Nickel-chromium alloys



United States Patent 3,248,213 NlCKEL-CHRflhllUlld ALLQYS Ronald A. Smith, West Hagley, and Edward G. Richards, Birmingham, England, assignors to The International Nickel Company, Inc, New York, N.Y., a corporation of Delaware Filed Nov. 21, 1962, Ser. No. 239,299 Claims priority, application Great Britain, Nov. 21, 1961, 41,629/61; Apr. 17, 1962, 14,380/62 9 Claims. (Cl. 75-171) The present invention relates to alloys and, more particularly, to nickel-chromium alloys useful at elevated temperatures in the form of sheet.

Alloys for use at elevated temperatures in the form of sheet, for example, in making fabricated articles such as casings and jet pipes for aircraft gas turbine engines, must not only retain adequate tensile and creep strength at the operating temperatures but must also be ductile at these temperatures and must be capable of being welded and formed at room temperature to the desired shape. The creep strength may be regarded as adequate if under a load of about 8 tons per square inch the creep elongation in 100 hours at the operating temperature does not exceed 0.2%, and preferably, it should not exceed 0.1%.

T he present specification is illustrated by the drawings in which:

FIGURE 1 is a graph showing the relationship between total aluminum and titanium content of alloys of the general kind in question and room temperature hardness of annealed sheet samples of said alloys; and

FIG. 2 is a graph showing the relationship between the ratio of titanium to aluminum and the tensile elongation of alloys both in accordance with and outside the present invention.

It is known that nickel-chromium-cobalt fbase alloys hardened by additions of titanium, aluminum, molybdenum and carbon have excellent high-temperature creep strength. However, the strength of alloys of this type falls progressively with increase in temperature. The following results indicate how one of the best currently available commercial sheet alloys, the nominal composition of which is 0.04% carbon, 20% chromium, 20% cobalt, 6% molybdenum, 2.2% titanium, 0.5% aluminum, the balance nickel, loses strength as the temperature is raised above 800 C. The tests were carried out on samples .of the alloys in the form of 0.048 inch thick sheet specimens after heat treatment by solution heating for 10 minutes at 1150 C. followed by air cooling, aging for 16 hours at 780 C. and again air cooling.

TABLE I Temperature, Stress, Creep in 100 C. t.s.i. hours, percent ice At the same time, however, the alloys become more difficult to form, as is shown by an increase in their hardness at room temperature in the solution heated and quenched condition. This effect is illustrated in FIGURE 1, in which the hardness is plotted against the total content of titanium and aluminum for a series of alloys containing, in addition to titanium and aluminum, 20% chromium, 10% to 20% cobalt, 0% to 10% molybdenum, 0.01% to 0.2% carbon, the balance being substantially all nick-e1. The alloys were tested in the form of sheet 0.048 inch thick that had been heated for 8 minutes at 1150 C. and water quenched. The low degree of scatter of the results plotted in thisfigure about the curve, indicates that variations in the contents of elements other than titanium and aluminum have only a very slight effect on room temperature hardness. A hardness of about 250 V.P.N. (Vickers Pyramid Number), which is the maximum generally acceptable for sheet alloys on the grounds of formability, is reached at a total titanium and aluminum content of 4%. 'For alloys to be used in sheet form, therefore, a limit is apparently set on the improvement in high temperature creep strength that can be gained by increasing the contents of titanium and aluminum.

Increasing the total content of titanium and aluminum also leads to a decrease in the high-temperature tensile ductility of the alloys and for this reason also an alloy that has adequate creep strength may not be suitable for use in the form of sheet. Although many attempts have been made to provide an alloy for sheet applications having an optimum combination of formability, weldability and ductility together with adequate creep strength at temperatures in excess of about 800 C., none, as far as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

The present invention is based on the discovery that both the high-temperature strength and the tensile ductility of nickel-chromium-cobalt alloys having a total titanium and aluminum content of about 4% are markedly influenced by the molybdenum content and the carbon content and surprisingly also by the ratio of the titanium to the aluminum content. Thus, by controlling these factors within specific limits, we are able to obtain alloys that have adequate strength and ductility at temperatures above 800 C. without an unacceptably high level of room temperature hardness so that they can be formed by conventional techniques.

It is an object of the present invention to provide an alloy suitable for use at high temperature in the form of sheet and having an optimum combination of strength, formability, toughness, weldability, etc.

Other objects and advantages will become apparent from the following description.

Alloys according to the invention contain, in percent by weight, from 14% to 20% chromium, from 10% to 16% cobalt, from 6% to 8% molybdenum, from 0.02% to 0.07% carbon, titanium and aluminum in such amounts that the total content of titanium and aluminum is from 3.5% to 4.5% and the ratio of titanium to aluminum is from 0.75:1 to 1.5: l, the aluminum content being at least 1.7%, from 0.001% to 0.005% boron and from 0% to 0.06% zirconium, the balance, apart from impurities, being nickel. To obtain the highest creep strength, the total content of titanium and aluminum should be at least 3.9% and the aluminum content at least 2%. The highest TizAl ratio consistent with these additional conditions is 1.25:1. The carbon content is advantageously from 0.03% to 0.06% and the boron content from 0.002% to 0.004%. Although the presence of zirconium benefits the creep strength and ductility of the alloys it has an adverse effect on the weldability. Therefore,

the zirconium content does not exceed 0.02% and no addition of zirconium is made during the melting of the alloys. The impurities commonly present in alloys of this type are silicon, manganese and iron. The amount of these elements should be as small as possible, and preferably, the content of each of them does not exceed 1% and the total content of all three does not exceed 1.5%.

The properties of the alloy, in particular the high temperature ductility and weldability, are reduced by trace amounts of other elements which can be introduced into the alloy from impure raw materials. In order to obtain the best possible properties, therefore, it is desirable to minimize the content of such trace elements. This can be achieved by using extremely pure raw materials but in commercial practice it is best effected by melting the alloys under a high vacuum and casting them under vacuum or in an inert gas atmosphere. Some benefit may also be obtained by refining the alloys by holding in the molten state under vacuum, for example, by beating them at 1400 C.-l600 C. for at least Sand preferably at least minutes under a pressure not exceeding 0.1 mm. Hg.

The alloys are most readily formed in the solution heated and quenched condition. Solution heating is preferably carried out in the temperature range 1050" C.- 1200 C. The duration of the heating will depend on the section size and may be from 2 to minutes for sections up to inch and from 2 to 8 hours for thicker sections. To obtain the minimum hardness, the alloys should then be cooled as rapidly as possible and water quenching is preferred. To develop the greatest high temperature strength, the alloys should then be aged at a temperature in the range 700 C.950 C., and preferably above 850 C., for from 2to 16 hours.

Welding of the alloys should be carried out when they are in the solution heated condition. No further solution treatment is in general necessary after welding but where maximum strength is desired, the aging treatment may be performed after welding, the temperature employed preferably being above 850 C., in order to ensure complete stress relief.

In order to achieve the desired combination of properties, it is essential that the contents of all the elements molybdenum, titanium, aluminum and carbon are within the ranges set forth above.

The effects of varying the molybdenum content on high temperature tensile strength and ductility are shown by the results set out in Table II, which were obtained with alloys having the nominal composition 20% chromium, 14% cobalt, 2% titanium, 2% aluminum, 0.04% carbon, 0.003% boron, balance nickel, in the form of sheet 0.048 inch thick which had been solution heated for 8 minutes at 1150 C., water quenched and aged for 16 1 0.1% elongation.

With increasing contents of molybdenum, the ultimate tensile stress and yield stress increase progressively but the elongation passes through a maximum and then decreases. The combination of maximum tensile strength with maximum tensile ductility is obtained with alloys containing 6% to 8% molybdenum.

Increase in the carbon content results in decreased tensile and creep strength. The results given in Tables III and IV show the effect of carbon content on the high temperature tensile properties and creep properties, respectively, of alloys of nominal composition 20% chromium, 14% cobalt, 2% titanium, 2% aluminum, 5% molybdenum, 0.003% boron, balance nickel. All the alloys were tested in the form of sheet 0.048 inch thick that had been solution heated for 8 minutes at 1150 C., water quenched and then aged. The aging treatment consisted of heating for 16 hours at 780 C. except in the case of the first alloy in TableIII which was aged for 4 hours at 900- C.

TABLE III Tensile properties at 840 0.

Carbon content,

percent U.T.S., t.s.i. Yield stress, Elongation,

t.s.i. percent TABLE IV Carbon content, Temp, C. Stress, t.s.i. Creep in percent hours, percent From these results, the advantage of keeping the carbon content below 0.07% is clear. If, however, the carbon content is below 0.02% the high temperature ductility is greatly reduced and there is a marked tendency for the alloys to develope very large grains during annealing which results in a poor quality in the form component and may lead to reduction in high temperature strength. While it has been found in practice that at least 0.02% carbon is generally present in the alloys, care should nevertheless be taken that the carbon content does not fall below this value.

In all high temperature sheet materials it is desirable to have the maximum high temperature tensile ductility consistent with the required level of strength. The importance of the ratio of the titanium to the aluminum content in achieving this combination of properties in the alloys of the invention is illustrated by FIGURE 2, in which the tensile elongation at 800 C. is plotted against the titanium/ aluminum ratio for alloys of nominal composition 20% chromium, 14% cobalt, 4% titanium plus aluminum, 5% molybdenum, 0.003% boron, balance nickel. Maximum ductility is obtained with alloys having a titanium/ aluminum ratio of about 1.0.

The boron content of the alloys is also import-ant. At least 0.001% and preferably 0.002% of boron must be present in the alloys to ensure adequate tensile ductility in'welded material. On the other hand, it is most desirable that sheet material should not crack on welding, and for this reason the boron content must not exceed 0.005% .and is preferably not more than 0.004%. The presence of excessive amounts of calcium or-magnesium also impairs weldability and for optimum weldability care should, therefore, be taken that the total content of these two elements is as low as possible, preferably not more than 0.005% or even 0.003%.

The importance of the correct relationship of the composition variables is further illustrated by the results of tests carried out on six alloys having the compositions set forth in Table V. Alloys Nos. 3 and 4 are in accordance with the invention while Nos. 1, 2, 5 and 6 are not.

TABLE V Composition, percent 1 Cr Mo Ti Al B Zr Si Mn 1 The balance of each alloy is substantially all nickel.

Each of the alloys was rolled into sheet 0.048i11ch thick These results as set forth in Tables VI, VII and VIII and samples of each of the alloys were annealed (Nos. 1 show that Alloy No. 1, in which the molybdenum conto 4 for 10 minutes at 1180 C. and Nos. 5 and 6 for 12 minutes at 1177 C.) and water quenched. They were then found to have the following hardnesses.

TABLE VI Alloy No.2 Hardness (V.P.N.) 1 332 Further sheet samples of each alloy were annealed and Water quenched in the same Way and were then aged by heating for 4 hours at 900 C. and air cooled to room temperature. The results of creep tests at 840 C. under a load of 8.6 t.s.i. and of tensile tests at a range of temperatures up to 900 C. are set forth in Tables VII and VIII, respectively.

TABLE VII Strain in 100 hours at 8.6 t.s.i. and 840 C.,

Alloy No.1 percent 1 0.07

1 Strain at 7.8 t.s.i. and 785 C.

TABLE VIII Alloy No Temp, U.I.S., Yield EL,

C. t.s.i. stress, t.s.i. percent tent was low, the carbon content was high, the total content of titanium and aluminum was high and the titanium to aluminum ratio was low, has an acceptable level of creep strength but is very hard in the annealed condition and has very poor tensile ductility at high temperatures.

Alloy No. 2, which differs from the alloys of the invention only in that the titanium content and the titanium to aluminum ratio are both too low, is readily softened on annealing and has very good tensile ductility but the creep strength of this material is poor.

In Alloy No. 5, the molybdenum and titanium contents are higher than those of alloys according to the invention, the aluminum content is lower and the titanium to aluminum ratio and total content of titanium and aluminum are both too high. While the hardness and creep strength of this alloy are acceptable, its tensile ductility is substantially lower than the alloys according to the invention.

Alloy No. 6 is very similar in composition to Alloy No. 5, and differs from the alloys according to the invention also in that its carbon content is too high and its boron content too low. These departures from the invention, although numerically small, result in-an unacceptably low creep strength, high hardness and low tensile ductility.

The invention specifically includes the alloys in the sheet form and also articles and parts fabricated therefrom by cold forming and by welding.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. As a new article of manufacture, a metal sheet formed from a nickel-chromium-cobalt base alloy characterized by a combination of (1) good tensile and creep strengths and ductility at elevated temperatures in excess of 800 C., the percentage of creep strain being not greater than 0.2% under a stress of 8 long tons per square inch at 800 C., (2) good weldability at room temperature, and (3) a hardness at room,temperature of not greater than 250 V.P.N. when cooled from solution treatment to thereby provide good formability characteristics, said alloy consisting essentially of from 14% to 20% chromium, from 10% to 16% cobalt, from 6% to 8% molybdenum, from 0.02% to 0.07% carbon, titanium and aluminum in such amounts that the total content of titanium and aluminum is from 3.5% to 4.5% and the ratio of titanium to aluminum is from 0.75:1 to 15:1, the aluminum content being at least 1.7%, from 0.001% to 0.004% boron, and the balance essentially nickel.

2. The article of manufacture in accordance with claim 1 in which the carbon content of the alloy set forth therein is from 0.03% to 0.06% and the boron content is from 0.002% to 0.004%.

3. As a new article of manufacture, a metal sheet formed from a nickel-chromium-cobalt base alloy characterized by a combination of (1) good tensile and creep strengths and ductility at elevated temperatures in excess of 800 C., the percentage of creep strain being not greater than 0.1% under a stress of 8 long tons per square inch at 800 C., (2) good weldability at room temperature, and (3) a hardness at room temperature of not greater than 250 V.P.N. when cooled from solution treatment to thereby provide good formability characteristics, said alloy consisting essentially of from 14% to 20% chromium, from to 16% cobalt, from 6% to 8% molybdenum, from 0.03% to 0.06% carbon, titanium and aluminum in such amounts that the total content of titanium and aluminum is from 3.9% to 4.5% and the ratio of titanium to aluminum is from 0.75 :1 to 1.25: 1, the aluminum content being at least 2%, from 0.002% to 0.004% boron, zirconium in an amount sutficient to enhance the creep strength and ductility of the alloy with the upper limit being 0.02% to thereby avoid detrimentally affecting the weldability characteristics of the alloy, the balance being essentially nickel.

4. A nickel-chromium-cobalt base alloy characterized by a combination of (1) good tensile and creep strengths and ductility at elevated temperatures in excess of 800 C., the percentage of creep strain being not greater than 0.1% under a stress of 8 long tons .per square inch at 800 C., and (2) good weldability at room temperature, and (3) a hardness at room temperature of not greater than 250 V.P.N. when cooled from solution treatment to thereby provide good formability characteristics, said alloy consisting essentially of about 17.7% to 20% chromium, about 13.3% to 14% cobalt, about 7.23% to about 8% molybdenum, about 0.026% to about 0.05% carbon, about 2% to about 2.1% titanium, about 1.75% to about 2.16% aluminum, about 0.003% to about 0.004% boron, zirconium in an amount sufilcient to enhance the creep strength and ductility of the alloy with the upper limit being 0.02% to thereby avoid detrimentally affecting the weldability characteristics of the alloy, the balance essentially nickel.

5. As a new article of manufacture, a metal sheet formed from the alloy set forth in claim 4.

6. A nickel-chromium-cobalt base alloy characterized and ductility at elevated temperatures in excess of 800 C., the percentage of creep strain being not greater than 8 800 C., (2) good weldability at room temperature, and (3) a hardness at room temperature of not greater than 250 V.P.N. when cooled from solution treatment to thereby provide good formability characteristics, said alloy consisting essentially of from 14% to 20% chromium, from 10% to 16% cobalt, from 6% to 8% molybdenum, from 0.02% to 0.07% carbon, titanium and aluminum in such amounts that the total content of titanium and aluminum is from 3.5% to 4.5% and the ratio of titanium to aluminum is from 0.75:1 to 1.5 :1, the aluminum content being at least 1.7%, from 0.001% to 0.004% boron, zirconium in an amount sufficient to enhance the creep strength and ductility of the alloy with the upper limit being 0.02% to thereby avoid detrimentally affecting the weldability characteristics of the alloy, the balance essentially nickel.

7. The alloy as set forth in claim 6 in which the carbon content is from 0.03% to 0.06%.

8. As a new article of manufacture, a cold-formed and welded article subjected in use to stress at temperatures in excess of 800 C. and made of the alloy set forth in claim 6.

9. As a new article of manufacture, a cold-formed and welded article subjected in use to stress at temperatures in excess of 800 C. and made of the alloy set forth in claim 7.

References Cited by the Examiner UNITED STATES PATENTS 2,766,155 10/1956 Bretteridge et a1. -171 2,920,956 1/1960 Nisbet et a1. 75-171 2,945,758 7/1960 Jahnke et a1. 75-171 3,008,855 11/1961 Swenson 75-171 3,047,381 7/1962 Hanink et al. 75-171 3,065,072 11/1962 Gittus et a1 75-171 3,094,414 6/1963 Franklin et a1. 75-171 3,107,167 10/1963 Abkowitz et al. 75-171 FOREIGN PATENTS 166,814 2/195 6 Australia. 548,777 12/ 1957 Canada.

DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, Examiner. by a combination of (.1) good tensile and creep strengths 0.2% under a stress of 8 long tons per square inch at R. W. GASS, C. M. SCHUTZMAN, R. O. DEAN,

Assistant Examiners. 

1. AS A NEW ARTICLE OF MANUFACTURE, A METAL SHEET FORMED FROM A NICKEL-CHROMIUM-COBALT BASE ALLOY CHARACTERIZED BY A COMBINATION OF (1) GOOD TENSILE AND CREEP STRENGTHS AND DUCTILITY AT ELEVATED TEMPERATURES IN EXCESS OF 800*C., THE PERCENTAGE OF CREEP STRAIN BEING NOT GREATER THAN 0.2% UNDER A STRESS OF 8 LONG TONS PER SQUARE INCH AT 800*C., (2)GOOD WELDABILITY AT ROOM TEMPERATURE, AND (3) A HARDNESS AT ROOM TEMPERATURE OF NOT GREATER THAN 250 V.P.N. WHEN COOLED FROM SOLUTION TREATMENT TO THEREBY PROVIDE GOOD FORMABILITY CHARACTERISTICS, SAID ALLOY CONSISTING ESSENTIALLY OF FROM 14% TO 20% CHROMIUM, FROM 10% TO 16% COBALT, FROM 6% TO 8% MOLYBDENUM, FROM 0.02% TO 0.07% CARBN, TITANIUM AND ALUMINUM IN SUCH AMOUNTS THAT THE TOTAL CONTENT OF TITANIUM AND ALUMINUM IS FROM 3.5% TO 4.5% AND THE RATIO OF TITANIUM TO ALUMINUM IS FROM 0.75D1 TO 1.5:1, THE ALUMINUMCONTENT BEING AT LEAST 1.7%, FROM 0.001% TO 0.004% BORON, AND THE BALANCE ESSENTIALLY NICKEL. 